The human cranial vault contains the brain, blood vessels, and cerebrospinal fluid (CSF). The sutures of the cranium fuse by a year of age and the skull becomes a rigid structure. The architecture and physiology of the intracranial space allow for some compensation for additional intracranial volume such as hemorrhage, tumor, or excess CSF. When this compensatory capacity is exhausted, the contents act essentially as ideal fluids in a rigid container, making them subject to rapid rises in pressure when a relatively small volume of fluid is added. With sufficient rise in intracranial pressure (ICP), brain tissue is compressed and its blood supply is compromised resulting in brain damage and, if unchecked, death.
In the normal brain, CSF is secreted by tissue known as choroid plexus within cavities in the brain called ventricles. The CSF flows from the uppermost lateral ventricles through conduits into the more central third and then fourth ventricles, then flowing out of the brain to surround the spinal cord and brain. Ultimately, the CSF is absorbed on the outer surface of the brain by cells comprising the arachnoid villi. This is a continuous circulation, amounting to approximately 400 cc/day. Any interruption in CSF circulation can result in excess CSF within the intracranial space, a condition known as hydrocephalus. In mild cases, CSF fills the ventricles excessively and stretches the cells of the brain resulting in neurological dysfunction. In severe cases, the rise in ICP may be sufficient to result in brain damage or death.
The most common contemporary treatment of hydrocephalus is to divert the flow of CSF. CSF is diverted to a space in the body that has a large capacity to absorb it such as the peritoneum, pleura, or bloodstream. A shunt for CSF diversion typically consists of a synthetic tube placed through a hole drilled in the skull and passed through the brain and terminates in the desired drainage location. Lumboperitoneal shunting is also possible, which avoids the need to drill into the skull by instead draining CSF from the lumbar region of spinal column, but is at greater risk of siphoning. The shunt may be fitted with a valve designed to control pressure and flow as well as a device designed to retard over-drainage due to siphoning with upright posture.
Currently available shunt technology has several shortcomings. Valve technology is often inadequate to provide the optimal level of drainage. Under-drainage results in elevated ICP and over-drainage can result in headaches or hemorrhage due to collapse of the brain and tearing of surface blood vessels. Differential pressure based shunts, even with “anti-siphon countermeasures,” often do not adapt well to changes in posture, to fluctuating CSF production and ICP, or to changes in intracranial CSF dynamics over time. Patients with shunts and persistent headaches frequently present a challenge because it is unclear whether there is subtle over- or under-drainage. The simple externally adjustable valves available currently force the clinician to guess at the appropriate pressure setting and accept that the system cannot adapt to fluctuations in demand.
CSF siphoning occurs when patient position results in additional pressure in the shunt due to gravitational forces acting upon the fluid column within the shunt and its tubing. This excess pressure is exerted across the shunt's differential valve, causing it to activate and undesirably allow CSF fluid to flow. This unwanted parasitic flow can reduce patient quality of life and can lead to numerous serious life threatening conditions by excess removal of CSF from the patient's brain. A significant source of this unwanted siphoning is the error pressure, as viewed from the valve, generated by the force, or resulting pressure, produced by the weight of the CSF fluid contained within the shunt tubing between the proximal catheter, the shunt tubing, and the distal catheter exit. Such error pressure is at a maximum when the patient is in the upright position, whereby the shunt's tubing is in general alignment with the gravitational field, and conversely is at a minimum when the patient is in a supine position and the tube is perpendicular to the gravitational field. The taller the patient, the worse the siphoning effect generally is, and such siphoning occurs in generally any position other than lying down (i.e., the supine position).
In the case of lumboperitoneal shunting, the CSF contained within the spinal column is the source of the error pressure generation. The pressure generated by the weight of the CSF in the spinal column can easily exceed the set point of differential valves in the shunt and lead to siphoning. A patient's motion and position affects how much error pressure is generated; minimum siphoning occurs when the patient and the shunt's flow path are supine, and maximum siphoning occurs when upright. This parasitic siphoning is a reason why the less costly and less complex surgical procedure of lumboperitoneal shunt treatment is not a more prevalent treatment option for hydrocephalus. Thus, there exists a desire for an anti-siphon device that prevents over drainage of CSF fluid regardless of patient position or activity.